Wash the spinach well and cook it in the water that clings to the leaves until barely soft. Drain and chop it finely or force it through a sieve or food mill.

Melt the butter, add the spinach, and cook for two or three minutes. Add salt, pepper and nutmeg to taste. Add the egg yolks and the cheese and mix.

Add the stock and reheat to boiling. The egg yolks will curdle, but they are intended to. Serve hot with croutons.